This invention generally relates ion implanters and more particularly to an apparatus and method for avoiding driver gas contamination in an ion implanter gas supply module.
Ion beam implanters are used to implant or xe2x80x9cdopexe2x80x9d silicon wafers with impurities to produce n or p type doped regions on the wafers. The n and p type material regions are utilized in the production of semiconductor integrated circuits. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type material. If p type material is desired, ions generated with source materials such as boron, gallium or indium are typically used.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The beam is formed and shaped by apparatus located along the beam path en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
During ion implantation a surface is uniformly irradiated by a beam of ions or molecules, of a specific species and prescribed energy. The size of the wafer or substrate (e.g. 8 inches or greater) is typically much larger than the cross-section of the irradiating beam which deposits on the wafer as a spot or xe2x80x9cribbonxe2x80x9d of about 1 inch. Commonly, in high current machines, the required uniform irradiance is achieved by moving the wafer through the beam.
Operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to surfaces of the implanter beam forming and shaping structure adjacent the ion beam path and also on the surface of the wafer support facing the ion beam. Contaminant materials also include undesirable species of ions generated in the ion source, that is, ions having the either the wrong atomic mass or undesired ions of the same atomic mass.
In a conventional ion implanter, an ion beam is emitted from an ion source and passed through a pre-analyzing magnet to remove undesired types of ions. Ions having identical energies but different masses experience a different magnetic force as they pass through the magnetic field due to their differing masses thereby altering their pathways. As a result, only those desired ions of a particular atomic mass unit (AMU) are allowed to pass through a prepositioned orifice in the pre-analyzing magnet.
After passing through the pre-analyzing magnet the ion beam is accelerated to a desired energy by an accelerator. Negative ions are changed into positive ions by a charge exchange process involving collisions with a chemically inert gas such as argon. The positive ions then pass through a post-analyzing magnet and finally reach a wafer where they impact the wafer and are implanted.
Ion implantation has the ability to precisely control the number of implanted dopant atoms into substrates to within 3%. For dopant control in the 1014-1018 atoms/cm3 range, ion implantation is superior to chemical diffusion techniques. Heavy doping with an ion implanter, for example, can be used to alter the etch characteristics of materials for patterning. The implantation may be performed through materials that may already be in place while other materials may be used as masks to create specific doping profiles. Furthermore, more than one type of dopant may be implanted at the same time and at the same position on the wafer. Other advantages include the fact that ion implantation may be performed at low temperature which does not harm photoresist and in high vacuum which provides a clean environment.
With respect to impurities generated in an ion implanter, among the most troublesome are those where the product of the mass M and the energy E is the same as that of the desired species in the ion beam. In such cases, since the impurities have the same radius of curvature as the desired ion beams, they are likely to pass through both the pre-analyzing and the post-analyzing magnet and reach the wafer.
In such cases there is frequently no way to remove impurities before they reach the wafer. The passage of even a small amount of impurities can have substantial degrading effects on the electrical characteristics of the wafer. For example, in the manufacture of gate oxide films, even if only a very small amount of undesired impurities reach the wafer the quality of a gate oxide film is degraded and in subsequent processing may cause the gate oxide film to grow to an undesired thickness. As a result, semiconductor device reliability is reduced.
One particularly troublesome impurity is N2 especially when carrying out an ion implantation process with silicon ions. Since Silicon and N2 have the same atomic mass unit (AMU) of 28 they are not differently affected or distinguished when passing through the pre-analyzing and the post-analyzing magnet. As a result, both species are passed through to the wafer, the N2 adversely affecting silicon implantation.
In an example where the presence of the impurity N2 can undesirably affect the performance of an ion implanter is in the calibration of the ion implanter by the use of a metrology instrument known as a thermawave to detect ion implantation damage in the target material. Generally, a measured dose of an implanted test species (measured by monitoring a physical property change in the implanted material) is compared with a previously recorded dose to determine the calibration state of the ion implanter. Consistency between test ion implantations with low mass ions may be used to provide information about the proper operation of the ion implanter. Silicon is frequently used as a test species that is implanted, causing measurable implantation damage which is subsequently measured by a thermawave tool. Generally, the thermawave tool measures a change in the surface reflectivity of the target material which corresponds to a known dose of implanted species. Comparing a present dose to a previously recorded dose indicates whether the ion implanter is performing properly within specifications. Clearly, where the impurity N2 reaches the wafer together with silicon ions in a calibration state test procedure, the calibration state will be altered resulting in faulty information concerning the operation of the ion implanter. For example, the thermawave results may erroneously indicate that the ion implanter is operating outside specifications.
One source of N2 as an impurity can arise in the case where gaseous sources of implantation material are in gaseous communication with a source chamber whereby pneumatic valves are used to select and deliver the source material to the source chamber for subsequent ionization. Frequently, due to lower cost or higher availability, N2 is used to drive operation of the various pneumatic valves used to deliver gaseous source materials to the source chamber. If a leak develops in the pneumatic valve, some amount of the N2 may leak into the into the source material and carrier gas (e.g., Argon) pathway, thereby contaminating the source material and ultimately leading to wafer contamination in the case where silicon is used as an implanting ion.
There is therefore a need to eliminate N2 contamination, especially in the case where silicon is used as an implanting ion.
It is therefore an object of the invention to eliminate the problem of N2 contamination, especially in the case where silicon is used as an implanting ion.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a method and apparatus for eliminating a contamination problem presented by a driver gas.
In one embodiment according to the present invention, a gaseous supply system for supplying at least one gaseous source material to an ion source chamber for use with an ion implanter includes at least one gas supply module in gaseous communication with an ion source chamber including at least one pneumatic valve to control the delivery of at least one gaseous source material to the ion source chamber for generation of source material ions for implantation; at least one gaseous source material in communication with the gas supply module for independent delivery of said at least one gaseous source material to the source chamber; and, a driver gas source in communication with said at least one pneumatic valve for operating the at least one pneumatic valve said driver gas source having a different atomic mass unit than the source material ions for implantation generated from the at least one gaseous source material.
In related embodiments, the driver gas source includes at least one inert gas, preferably helium and argon. Further, the at least one gaseous source material includes at least one gaseous source of ions selected from the group of As, Ph, B and Si.
In another embodiment, the at least one gas supply module with the at least one pneumatic valve includes a high pressure valve in downstream communication with the at least one gaseous source material said high pressure valve in upstream communication with a means for determining a flow rate and a low pressure select valve in downstream communication with the means for determining a flow rate for selecting gaseous downstream communication with an ion source chamber. Further, the a low pressure bypass valve is in upstream gaseous communication with said high pressure valve and in gaseous downstream communication with said downstream ion source chamber to define a gaseous pathway bypassing said means for determining a flow rate and said low pressure select valve.
In yet another embodiment, a plurality of gas supply modules is in parallel gaseous communication with a downstream ion source chamber.
In another aspect of the invention, the gas supply system includes a purge line including at least one pneumatic valve in communication with said gas supply module wherein the purge line is fed with a gaseous purge source having a different atomic mass unit than the source material ions for implantation generated from the at least one gaseous source material.
In another aspect according to the present invention a method for using the apparatus of the first embodiment is presented.
These and other objects, advantages and features of the invention will become better understood from a detailed description of a preferred embodiment of the invention which is described in conjunction with the accompanying drawings.